When Do You Need OTDRs?

In outside plant fiber optic installations, every cable installed will be tested for end-to-end loss with a source and power meter and an OTDR trace will be taken of each fiber. Yet OTDRs—optical time domain reflectometers—are rarely used in premises applications. Why is that?

The usual analogy is that an OTDR is like radar—it sends out a high-powered pulse and analyzes the returned signal to create a “picture” of the cable.

The returned signal on OTDRs is backscattered light. Backscattering is caused by the small amount of light scattered by the atoms and molecules in the glass of the fiber that causes most of the loss in the fiber. Part of that backscattered light returns back up the fiber to the source. The backscatter signal is very small, about one-millionth of the outgoing signal, but good optical and electronic design makes it possible to capture and analyze that signal.

Since the pulse from the OTDR is attenuated as it goes down the fiber and again as it comes back after backscattering, the OTDR can calculate the losses in the fiber. The resulting data can be made into a nice picture of what’s going on in the fiber, called a “trace” or “signature.” The OTDR can measure fiber length, loss, connector and splice location and loss, and even find losses caused by installation stress. Since it works on signals scattered back toward the instrument, it only requires access to one end of the fiber.

Sounds too good to be true, doesn’t it? Well, OTDRs have some drawbacks.

OTDRs are more qualitative than quantitative instruments. The losses they measure are calculated from the backscatter signature, not tested directly as with meters and sources. Changes in fiber characteristics like attenuation coefficient or core diameter create directional variances that require measurement from both ends to get a trustworthy measurement.

Since the imputed value from the OTDR test never agrees with end-to-end loss measurements made with a source and power meter (optical loss test set or OLTS), no industry standard allows OTDR readings to be used instead of end-to-end loss testing for cable certification.

The outside plant guys in telecomm or CATV depend on their OTDRs. The OTDR allows them to verify that every splice was properly made and that no damage to the cable from excess stress was done during installation. If there is a cable failure, which is usually caused by “backhoe fade” (I bet I don’t need to explain that), the OTDR can find the broken end of the cable and measure the distance to the break. That speeds up restoration immensely.

Using OTDRs in premises cabling is not so common. Most cables are too short to require splicing, but instead are terminated directly with connectors. Typical OTDRs are optimized for outside plant cabling consisting of long lengths of fiber. For a short premises cable plant, the typical OTDR would be of limited usefulness, but high-resolution OTDRs made just for the short distances of premises applications have started to become available.

Unlike meters and sources, where the data is presented as a straightforward digital number, OTDR data comes mainly from interpreting the traces, since much of the useful data is in the picture.

Most OTDRs offer an “autotest” feature that generates loss and length numbers, but these instruments are simply not smart enough to find many common problems like “ghosts” caused by reflections in short cables or “gainers” caused by joints in different fibers. A “pass/fail” from an OTDR is simply not acceptable.

I have personal experience with several jobs where installers misused OTDRs and it cost them a lot of time and money. On one job, the installers pulled out $100,000 worth of perfectly good cabling they had installed and threw it in the trash because they did not know how to interpret the OTDR traces. They only used the digital autotest data, which erroneously told them the cable was bad.

Put a OTDR in the hands of an untrained installer and you may be in the middle of a lot of disputes between contractors, end users and vendors. OTDR users must have extensive training in using the instrument, interpreting traces and understanding the limitations of the instrument. Basic OTDR training takes at least two or three days, if the operator already has a strong background in fiber.

The biggest deterrent to use of OTDRs in premises applications is cost. While a source and meter can be bought for under $1,000, the cheapest OTDRs sell for about $15,000, unless you choose the one that uses a laptop for display at about half the price. Add in both single-mode and multimode capability and your cost nears $25,000.

Amortizing that cost over the fibers you install and charging it to the customer may make your prices uncompetitive. But when the customer wants it, and is willing to pay for it, you need to know how to use the OTDR correctly. EC

HAYES is a VDV writer and trainer and the president of The Fiber Optic Association. Find him at www.JimHayes.com.